RSR13 is a synthetic allosteric modifier of hemoglobin. It reduces hemoglobin-oxygen binding affinity and enhances the diffusion of oxygen from the blood to the tissues. Hypoxic conditions are present in tumors. This may be a major cause of treatment failure since hypoxic cells are substantially more resistant to radiation than oxygenated cells. RSR13 may decrease these hypoxic conditions, thereby increasing the efficacy of radiation treatment. We are proposing a phase II study to evaluate the efficacy and safety of RSR13 combined with cranial irradiation in the treatment of brain metastasises.